Heredity and Genetics Part Two Dihybrid Crosses

1
Heredity and GeneticsPart TwoDihybrid Crosses
2
Review of Monohybrid Crosses

• Remember, monohybrid crosses involve only ONE
  trait
• Practice In fruit flies, red eyes are dominant
  over white eyes.
• Cross a white-eyed fly with a homozygous dominant
  red-eyed fly.
• Cross two heterozygous red-eyed flies.
• Draw a Punnett square for each cross,
• and determine the genotypic and phenotypic ratios.

3
Review of Monohybrid Crosses

• Can you determine the genotype
• of a white-eyed fly just by looking at it?
• Yes- its rr
• Can you determine the genotype
• of a red-eyed fly just by looking at it?
• No- it could be RR or Rr
• How could you determine the genotype of the
  red-eyed fly?
• Perform a test cross

4
Review of Monohybrid Crosses

• What two organisms would you cross in the test
  cross?
• The unknown red-eyed fly with a white eyed fly
• If the red-eyed fly was RR, what would the
  offspring of the test cross look like?
• All of the offspring would have red eyes
• If the red-eyed fly was Rr, what would the
  offspring of the test cross look like?
• Half of the offspring would have red eyes, half
  would have white eyes

5
Dihybrid Crossa cross that shows the possible
offspring for two traits
Coat Texture R Rough r Smooth
Fur Color B Black b White
In this example, we will cross a heterozygous
individual with another heterozygous individual.
Their genotypes will be
BbRr x BbRr
6
Dihybrid Cross

• BbRr x BbRr
• First, you must find ALL possible gametes that
  can be made from each parent.
• Remember, each gamete must have one B and one R.

7
Dihybrid Cross

• BbRr x BbRr
• Possible gametes
• BR
• Br
• bR
• br

Next, arrange all possible gametes for one parent
along the top of your Punnett Square, and all
possible gametes for the other parent down the
side of your Punnett Square
8
Dihybrid Crossesa cross that shows the possible
offspring for two traits
BbRr x BbRr
Fur Color B Black b White
Coat Texture R Rough r Smooth
Then, find the possible genotypes of the offspring
9
Dihybrid Crossesa cross that shows the possible
offspring for two traits
BbRr x BbRr
Fur Color B Black b White
Coat Texture R Rough r Smooth
10
How many of the offspring would have a black,
rough coat? How many of the offspring would have
a black, smooth coat? How many of the offspring
would have a white, rough coat? How many of the
offspring would have a white, smooth coat?
Fur Color B Black b White
Coat Texture R Rough r Smooth
11
How many of the offspring would have black, rough
coat? How many of the offspring would have a
black, smooth coat? How many of the offspring
would have a white, rough coat? How many of the
offspring would have a white, smooth coat?
Phenotypic Ratio 9331
Fur Color B Black b White
Coat Texture R Rough r Smooth
12
More Practice

• On the back of your notes
• In pea plants, yellow seeds (Y) are dominant over
  green seeds (y), and rounded peas (R) are
  dominant over wrinkled peas (r).
• Cross a plant that is heterozygous for both
  traits with a plant that is homozygous recessive
  for both traits. Draw a Punnett square to show
  all possible offspring, and determine the
  genotypic and phenotypic ratios.

13
YyRr X yyrr